Neurological System Notes

Basics of Neurology

• Nervous tissue consists of:
  • Neurons: Excitable cells for electrical or chemical information transmission.
  • Neuroglia: Supporting cells providing structural support, protection, and nutrition, aiding neurotransmission.

Neuron Anatomy

• Cell body: Contains the nucleus.
• Axon: Carries nerve impulses away from the cell body.
• Dendrites: Carry nerve impulses toward the cell body.
• Myelin: Lipid covering over axons to facilitate nerve impulse transmission.

Nerve Impulse

• Neurons communicate through electrical impulses (action potentials) traveling across the axon to the synaptic cleft.
• Action potentials reaching the synaptic cleft:
  • Spread the electrical impulse to the post-synaptic neuron.
  • Trigger the release of neurotransmitters.

Nerve Injury

• Injuries resulting in cell death are permanent.
  • Mature neurons do not divide to create new neurons.
  • Dead neurons undergo liquefactive necrosis (turn into fluid or pus-filled cysts).
• Injured but not dead neurons may slowly repair through axonal reaction.
  • Neurons may form new connections via neural plasticity.

CNS and PNS

• Nerves are organized into the central nervous system (CNS) and the peripheral nervous system (PNS).
• Central Nervous System (CNS):
  • Composed of the brain and spinal cord.
  • Contains relay neurons (interneurons).
• Peripheral Nervous System (PNS):
  • Composed of cranial nerves, spinal nerves, and peripheral nerves.
  • Contains sensory neurons and motor neurons.

Central Nervous System

• The central nervous system consists of the brain and spinal cord.

Brain

• Forebrain:
  • Frontal lobe: goal-oriented behavior, movement, short term memory
  • Parietal lobe: sensory input
  • Temporal lobe: auditory, speech recognition, long term memory
  • Occipital lobe: vision
  • Tectum: involved in certain reflex arcs
  • Cerebral aqueduct: part of the cerebrospinal fluid ventricle system
  • Tegmentum: helps maintain homeostasis
  • Cerebral peduncles: connect cerebrum to brainstem
• Hindbrain:
  • Cerebellum: balance, posture, fine motor movements
  • Pons: respiration
  • Medulla oblongata: respiration, heart rate, blood pressure, cough/gag, swallowing

Spinal Cord

• The spinal cord transmits long motor and sensory tracts from the brain and synapses with cell bodies in the grey matter of the spinal cord.
  • Afferent: towards CNS
  • Efferent: away from CNS

Meninges

• Three protective membranes covering the brain and spinal cord:
  • Dura mater
  • Arachnoid
  • Pia mater

Cerebrospinal Fluid

• Cerebrospinal fluid (CSF) is a clear, colorless fluid that functions to provide support and nutrients to brain tissue.
  • Provides buoyancy and prevents the brain from tugging on the meninges, nerve roots, and blood vessels.
  • Produced by the choroid plexuses and reabsorbed through the arachnoid villi.
• Too much CSF can exert pressure within the brain and spinal cord.

Peripheral Nervous System

• Peripheral nervous system (PNS):
  • 31 pairs of spinal nerves, named according to the vertebral level from which they exit.
  • 12 pairs of cranial nerves: Olfactory, Optic, Oculomotor, Trochlear, Abducens, Trigeminal, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal.
  • Peripheral nerves.

Autonomic Nervous System

• Contains cells from CNS and PNS and coordinates to maintain steady state among internal organs.
  • Sympathetic:
    • Fight or flight response.
    • Release of epinephrine and norepinephrine to vasodilate vessels to muscles and vasoconstrict vessels to viscera.
    • Increases blood pressure and heart rate.
  • Parasympathetic:
    • Rest and digest response.
    • Reduced heart rate and blood pressure; increased digestion and visceral function.

Pain and Temperature

Nociception

• The processing of harmful stimuli through the nervous system.

Nociception Phases

1. Transduction: Tissue damage from chemical, mechanical, or thermal noxious stimuli is converted to electrophysiologic activity.
2. Transmission: Conduction of pain impulses into the dorsal horn of the spinal cord and to the reticular formation, hypothalamus, thalamus, and limbic system.
3. Perception: Conscious awareness of pain.
4. Modulation: Process of increasing or decreasing transmission of pain signals.

Types of Nociception

• Acute:
  • Protective mechanism to alert an individual to an injury or condition that is immediately harmful to the body.
  • Pain <3 months.
  • Signs/symptoms: tachycardia, hypertension, diaphoresis, dilated pupils, anxiety.
• Chronic:
  • Lasts >3 months.
  • Serves no protective purpose.
  • May involve dysregulation of nociception modulation.
  • May cause behavioral and psychological changes such as depression and anxiety.
• Somatic Pain:
  • Muscles, bones, joints, and skin.
  • Pain is typically sharp and well localized but sometimes can be dull, throbbing and poorly localized
• Visceral Pain:
  • Organs and bodily cavities.
  • Pain is dull, aching, and poorly localized; often radiates or is referred
• Referred Pain:
  • Pain that is felt distant to the point of origin due to convergence on the same spinal neurons.
• Neuropathic Pain:
  • Nervous system dysfunction from improper modulation.
  • Hyperalgia and allodynia

Temperature Regulation

• Normal body temperature is achieved through precise balancing of heat production, heat conservation, and heat loss.
  • Normal range is $36.2$ to $37.7$ C ($97.2$ to $99.9$ F).
  • Controlled by the hypothalamus, which receives input from peripheral thermoreceptors and triggers heat conservation/production or heat loss.
• The hypothalamus may also trigger a fever (>$100.5$ F or $38$ C).
  • May be physiological
    • Benefits of stimulating a fever: kills microorganisms, deprives bacteria of food source.
  • May be pathological
    • Trauma or stroke causing injury to the hypothalamus.
• Heat loss:
  • Radiation.
  • Conduction (loss of heat through direct contact).
  • Convection (loss of heat through gasses or liquids).
  • Evaporation.
  • Vasodilation.
  • Decreased muscle tone.
  • Increased pulmonary ventilation.
  • Voluntary mechanisms.
• Heat conservation/production:
  • Vasoconstriction.
  • Skeletal muscle contraction (movement, shivering).
  • Chemical thermogenesis.
  • Chemical reactions of metabolism.
  • Voluntary mechanisms.

Alterations in Cognition

Altered Levels of Consciousness (ALOC)

• Consciousness requires arousal (wakefulness) and awareness (content of thought).
• Altered levels of consciousness are typically signs of bigger problems:
  • Neurologic (stroke, seizure, trauma, tumor, intracranial infection, progressive neurological disease).
  • Metabolic (infection, organ failure, hypoglycemia, drug interactions or overdoses).
  • Psychogenic (underlying psychiatric illness).

Levels of Altered Consciousness

• Confusion: Impaired judgement and decision making; cannot think rapidly or clearly.
• Disorientation: Considered the beginning of loss of consciousness.
  • Test orientation to person, place, and time.
• Lethargy: Mild decrease in arousal. Awakens upon light stimulation but limited spontaneous movement.
• Obtundation: Moderate decrease in arousal. Falls asleep unless repeatedly stimulated.
• Stupor: Severe decrease in arousal. May open eyes in response to vigorous or noxious stimulus.
• Light coma: Unresponsive but with purposeful movement upon stimulation. Eyes may be closed or open.
• Coma: No response to external environment except upon noxious stimulation.
• Deep coma: Unresponsive with no response to any stimuli.

Acute Encephalopathy

• General term encompassing acute confusional states.
• Patients present with sudden onset deficits in arousal and awareness.
  • May have fluctuating symptoms of hallucinations, delusions, restlessness/agitation.
• May be toxic metabolic (caused by disorders of the body leading to disruptions in brain function).
  • Illicit drug use, overdose, drug interactions, metabolic disorders.
• May be neurological in origin.
  • Trauma, cerebral edema, meningitis/encephalitis, seizures, stroke.
• Treatment includes identifying the cause and using pharmacological and nonpharmacologic supportive measures.

Delirium

• Can be considered a type of acute encephalopathy/acute confusional state.
• Reversible acute state of brain dysfunction.
  • Typically caused by metabolic disorders (drugs, dehydration, infection, sleep deprivation).
  • No primary intracranial pathologies.
• More commonly seen in older adults. The greatest risk factor is the severity of illness.
• Often seen in the intensive care unit or post-operatively; tends to develop after 2-3 days.
• Hallmark sign is waxing and waning symptoms.
  • Hyperactive delirium – agitation, aggression, restlessness.
  • Hypoactive delirium – drowsiness, confusion, apathy.
• Treatment: Supportive care including gentle reorientation, correction of disrupted sleep-wake cycle, review of medications, antipsychotics.

Dementia

• Progressive non-reversible failure of cerebral functions that causes impairment to arousal and awareness.
• Etiologies may include degeneration of neurons, compression of brain tissues, atherosclerosis of cerebral vessels, trauma, genetics, etc.
• The onset of symptoms is typically gradual and becomes worse with time; however, exact symptoms and progression pattern vary based on the type of dementia.

Alzheimer’s Dementia

• Most common type of dementia.
  • Full pathophysiology is unknown; however, risk increases with age and positive family history.
  • Tend to have a build up of cerebral amyloid protein plaques and tau protein bundles.
• Progresses from mild short-term memory deficits and culminates in a total loss of cognition and executive functions.
• Newer treatments may potentially halt disease progression.

Seizures

• Uncontrolled electrical activity in the brain that leads to disruption of neuronal activity.

• Seizures may have many different causes and are typically symptoms of an underlying disorder.

• Etiologies include: genetic disorders, metabolic disorders (drug intoxication or overdose, hypoglycemia, hypo or hypernatremia), brain tumor, trauma, infection, strokes, intracranial hemorrhage, or idiopathic.

• The term epilepsy is used to describe the condition of recurrent seizures.

• Symptoms of the seizure will depend on the location of the uncontrolled electrical activity and how far it spreads.

• Describing seizures based on location of abnormal electrical activity:

  • Generalized: Abnormal electrical activity encompasses both hemispheres of the brain; the patient loses consciousness.
  • Focal (partial): Abnormal electrical activity is restricted to only one hemisphere; the patient remains conscious.

• Describing seizures based on movement:

  • Tonic-clonic: Rhythmic periods of muscle contraction with increased muscle tone followed by relaxation.
  • Other types of movement: Tonic, atonic, clonic, myoclonic.

• Atypical seizures:

  • Absence seizures, aggression (frontal lobe), sensory hallucinations (parietal), visual hallucinations (occipital lobe), auditory hallucinations (temporal lobe), aphasia.

• After a seizure, many patients will enter a postictal state including confusion, drowsiness, headache, memory loss, weakness, etc as the brain tries to recover.

  • Postictal states typically last several minutes to hours.

• Status epilepticus:

  • Old definition: seizures >30 minutes
  • New definition: seizure lasting 5 minutes or longer, or >1 seizure within a 5-minute period without the patient returning to baseline.
  • Life-threatening condition; the patient needs emergency medical treatment.
  • Treatment includes seizure abortive medications (benzodiazepines, anti-seizure medications) and supportive care.

Cerebral Edema

• Swelling of brain tissue.
• Can be caused by traumatic brain injuries, intracranial lesions (tumors and strokes), or excessive CSF.
• Symptoms: agitation, confusion, drowsiness, coma.
• Leads to increased intracranial pressure:
  • Decreased cerebral perfusion and injury to surrounding brain tissue.
  • Eventual brain herniation if left untreated.

Brain Herniation

• Life-threatening condition that requires emergency treatment.
• Pressure inside the skull causes part of the brain to push against other structures.

Hydrocephalous

• Accumulation of cerebrospinal fluid in the brain.
• May lead to cerebral edema and increased intracranial pressure.
• Acute hydrocephalous may have acute symptoms similar to cerebral edema.
• Chronic hydrocephalous may have more mild symptoms.

Neurological Assessment

• Typical neurological assessments for wakefulness and consciousness would include evaluation of mental status.
  • Is the patient awake?
    • If the patient is not awake, are they able to awaken to stimulus?
    • Once they awaken, do they stay awake or do they require repeated stimulus?
  • Is the patient oriented to person, place, and time? Does the patient know why they are being evaluated?
  • How is the patient’s speech and understanding?
    • Does the patient follow commands?
    • Is the patient able to communicate?
    • Is the patient able to read and write?
• How do you test for a patient’s neurological status if they are in a stupor, light coma, coma, or deep coma?
  • Check for wakefulness – some patients will awaken from comatose states!
  • Check patterns of breathing.
  • Check eye opening.
  • Evaluate motor response.
  • Check cranial reflexes.
    • Pupillary reflex.
    • Corneal reflex.
    • Oculocephalic eyes.
    • Occulovestibular reflex.
    • Cough/gag reflex.

Patterns of Breathing

• Cheyne-Stokes respirations: abnormal breathing pattern characterized by periodic hyperventilation, hypoventilation, and then apnea.
  • Caused by bilateral injury to cerebral structures.
  • Ventilation is only stimulated once $PaCO<em>2$ is abnormally increased, slows down, and then stops once $PaCO</em>2$ drops.
  • Described as “crescendo-decrescendo” pattern.
• Injury to the pons or medulla oblongata may cause hyperventilation, apnea, and agonal breathing patterns.

Evaluation of Motor Response

• Purposeful movements:
  • Localization (movement towards noxious stimuli).
  • Withdrawal (movement away from noxious stimuli).
• Posturing:
  • Decorticate (flexion).
  • Decerebrate (extension).
• Absent.

Pupillary Reflex

• Pupillary changes can be an indication of brainstem function, involving cranial nerves II (optic) and III (occulomotor).
  • Normal pupils should be symmetrical in size and shape and respond to light.
  • Abnormal pupillary responses may be asymmetrical pupils, abnormal constriction or dilation, or lack of response to light.
• Sudden changes to pupil shape and fixed/dilated pupils should be a red flag.

Occulocephalic Reflex

• Also called the doll’s eyes reflex.
• Tests CN III, VI, VIII, and brainstem.
• Turn the head from side to side and watch the direction of the pupils.
  • Be careful in patients with neck injuries.
  • Oculocephalic reflex is present: eyes stay focused on midline.
  • Oculocephalic reflex is absent: eyes move in the same direction as the head (similar to a doll).

Occulovestibular Reflex

• Also called cold caloric testing.
• Tests CN III, VI, VIII, and brainstem.
• Cold water is inserted into the ear canal while the examiner watches the direction of pupillary movement and nystagmus.
  • Vestibulocochlear reflex is present if the patient looks towards the side of cold water injection. Nystagmus is in the opposite direction.
  • Vestibulocochlear reflex is absent. The patient’s eyes do not move and there is no nystagmus.

Brain Death

• Brain death is the complete and irreversible cessation of all brain activity including brainstem and cerebellum.
• Must meet strict diagnostic criteria to be declared brain death:
  1. Absence of confounding variables (no severe underlying metabolic disturbances, no CNS depressant drugs or paralytics, normal core body temperature, normal blood pressure).
  2. Clinical exam consistent with brain death (no movement, no sign of brainstem activity/absent cranial reflexes, no spontaneous respirations).
  3. Apnea tests consistent with brain death.
  4. Ancillary tests as desired (EEG, transcranial doppler ultrasound, HMPOA SPECT).
  5. If any criteria are not met, then brain death cannot be declared.
• Once brain death is declared, most states will declare that patient legally dead.

Brain Death Confusion

• Spinal reflex arcs may still be present, such as the “Lazarus sign”.
• Patients may continue to have cardiovascular function after they are declared brain death.
  • Many patients become hemodynamically unstable.
  • Patients that “survive” are dependent on “life”-sustaining measures such as ventilators, artificial nutrition, and certain medications from IV drips.
• Cases in which brain death is incorrectly diagnosed are extremely rare and often sensationalized or incorrectly reported.

Cerebral Death Confusion

• Brain death is NOT the same as cerebral death.
• Cerebral death refers to the cessation of cerebral (telencephalon or forebrain) functioning.
  • In cerebral death, the midbrain and hindbrain are still intact.
    • Cranial reflexes are present.
    • The patient has spontaneous respirations.
• This is not considered legal death.
• Patients typically progress to:
  • Persistent vegetative state: spontaneous eye opening without regaining consciousness, cannot follow commands, cannot voluntarily move, dependent on artificial nutrition.
  • Minimally conscious state: occasional brief periods of consciousness and limited movement.

Locked-In Syndrome

• NOT brain death.
• Inability to communicate either through speech or body movement.
• CN I-IV are typically preserved (able to blink and move eyes).
• Conscious and cognitive function are intact.
• Caused by pontine injuries (central pontine myelinolysis or pontine infarcts).